MIMO technology has attracted a lot of attention in areas of telecommunications because it allows a significant increase in throughput for any given bandwidth and overall power expenditure. MIMO technology exploits phenomena such as a multi-path propagation to increase data throughput and range, or to reduce bit error rates. In general, MIMO technology increases the spectral efficiency of a wireless telecommunications system.
FIG. 1 illustrates a conventional MIMO signal receiving apparatus 100, which includes two typical direct conversion receivers 102 and 104 coupled to two antennas 106 and 108, respectively. The direct conversion receiver 102 is comprised partially of a radio frequency (RF) filter 110, a low noise amplifier 112, a RF filter 114, a RF amplifier 116, and an IQ demodulator 118, which mixes outputs of the RF amplifier 116 and outputs of a RF local oscillator 119 to generate I and Q baseband signals that are forwarded to baseband low pass filters 120 and 122, respectively. The outputs of the baseband low pass filters 120 and 122 are amplified by variable gain amplifiers 124 and 126, respectively, to produce filtered I and Q signals for analog-to-digital converters (not shown in this figure) to convert into digital signals for further processing. The direct conversion receiver 104 has a functional structure similar to that of the receiver 102. Thus, the detailed description of the receiver 104 is hereby omitted in order to avoid redundancy.
The conventional direct-conversion type MIMO signal receiving apparatus is simple in design and has the advantages of low manufacturing cost and low power consumption. However, it is particularly susceptible to signal interference induced by adjacent out-of-band frequencies (so called jammers). This causes serious problems when such direct-conversion type MIMO signal receiving apparatus is implemented in a mobile device. For example, if the mobile device is moved to an area where there are many high power transmitters operating at frequencies that are very close to the receiver's desired channel, then the receiver will not function properly due to interference from the high power transmitters.
FIG. 2 illustrates a conventional superheterodyne MIMO signal receiving apparatus 200, which includes two typical superheterodyne receivers 202 and 204 coupled to two antennas 206 and 208, respectively. The superheterodyne receiver 202 is comprised partially of an RF filter 210, a low noise amplifier 212, an RF filter 214, an RF amplifier 216, and a mixer 218, which mixes outputs of the RF amplifier 216 and outputs of an RF local oscillator 220. An intermediate frequency surface acoustic wave (IF SAW) filter 222 receives the mixed signal output from the mixer 218 and output a filtered signal to a variable gain amplifier 224. An IQ demodulator 226 mixes outputs of the variable gain amplifier 224 with outputs of an IF local oscillator 228 to generate I and Q baseband signals for analog-to-digital converters (not shown in this figure) to convert into digital signals for further processing. The superheterodyne receiver 204 has a functional structure similar to that of the receiver 202. Thus, the detailed description of the receiver 204 is hereby omitted in order to avoid redundancy.
FIG. 3 illustrates another conventional superheterodyne MIMO signal receiving apparatus 300, which includes two typical superheterodyne receivers 302 and 304 coupled to two antennas 306 and 308, respectively. The superheterodyne MIMO signal receiving apparatus 300 is similar to the superheterodyne MIMO receiving apparatus 200 (shown in FIG. 2) in their functional structures, except that the apparatus 300 has two stages of RF-to-IF conversion, whereas the apparatus 200 has only one stage.
The above-mentioned conventional superheterodyne MIMO signal receiving apparatuses are superior to the direct-conversion type MIMO signal receiving apparatus 100 in terms of rejecting the out-of-band jammers, and therefore is able to receive signals with good quality in an area where there are many high power transmitters operating at the frequencies that are very close to the receiver's desired channel. However, the superheterodyne MIMO signal receiving apparatus 200 has disadvantages, such as high manufacturing costs, high power consumption rates, and a large equipment size, due to its additional devices that are needed for down converting RF signals received from the antennas into IF signals.